The present invention relates to an optical super-resolution technique for improving resolution beyond the theoretical resolution limit of an optical device well-corrected for aberration. The invention also relates to a technique for eliminating side lobes associated with super resolution. More particularly, the invention relates to a technique for improving the resolution of an optical pickup for an optical disk.
The theoretical resolution limit of an optical device will be briefly explained. In an optical device designed to be substantially free from aberration in geometrical optics, the spot image it produces is focused as an infinitely small optical spot. In reality, however, the optical spot exhibits a finite degree of spreading because of the diffraction arising as a consequence of the wave nature of light. Here, when the numerical aperture of the optical device, which contributes to the focusing or converging of the spot, is denoted by NA, the physical definition of the optical spot spreading is given by the formula kxc3x97xcex÷NA, where xcex is the wavelength of light, and k is a constant that depends on the optical device and usually takes a value between 1 and 2. The numeral aperture NA is, in general, proportional to D/f which is the ratio of the effective entrance pupil diameter, D, of the optical device (usually, the effective beam diameter) to the focal length, f, of the optical device.
Therefore, if the theoretical resolution of the optical device is to be increased, that is, if the optical spot is to be focused to a smaller diameter, either light of a shorter wavelength should be used or the numerical aperture NA should be increased.
However, the wavelength of a commonly used laser light source is 780 nm or 650 nm. In recent years, a laser light source with a wavelength of 410 nm has been developed, but a laser light source having a wavelength of 380 nm or shorter is either difficult to achieve or expensive to implement.
On the other hand, as the numerical aperture NA of an optical device increases, it becomes increasingly difficult to design the optical device to be free from aberration in geometrical optics. Further, the focal depth of the optical device decreases with the square of the numerical aperture NA, while the coma of the optical device increases with the cube of the numerical aperture NA. Under the current circumstances, therefore, designing an optical device with a numerical aperture NA of about 0.7 or larger is either difficult to achieve or expensive to implement. It should also be noted that the usual optical materials used to construct optical devices are opaque to light at 380 nm and shorter wavelengths. As a result, optical devices using such optical materials have the disadvantage that light cannot be effectively utilized.
For the reasons stated above, there are limitations in increasing the theoretical resolution of the optical device by using light of shorter wavelength or by increasing the numerical aperture NA.
In view of this, an alternative technique for further improving the above-described theoretical resolution of the optical device is proposed in xe2x80x9cO plus Exe2x80x9d (No. 154, pp. 66-72, 1992). This technique uses a light blocking plate to block a portion of the effective beam of a super-resolution optical device and to thereby make the optical spot size 10 to 20% smaller than the theoretical limit of the optical device. This technique achieves an effect equivalent to increasing the numerical aperture NA of the optical device or making the wavelength of the light source shorter.
However, the above technique has had the problem that when an optical spot is formed by the super-resolution optical device, side lobes or relatively large peaks peculiar to super resolution appear on both sides of the spot, making the optical spot appear as if it had three peaks.
This phenomenon will be explained with reference to FIGS. 4 and 5. FIG. 4 is a schematic diagram of an optical device for explaining the principle of super resolution. First, the aperture of a converging lens 406 is blocked using a blocking mask 403 of radius r with its center at an optical axis 401. It is assumed here that the radius r of the blocking mask 403 is smaller than the radius of an effective beam 402. FIG. 4 shows a cross-sectional view of the optical device, but it should be noted that the actual optical device has a shape that is rotationally symmetrical about its optical axis 401.
Here, the optical spot 501 formed at point P, i.e., the focal point of the converging lens 406, will be considered. The optical spot 501 can be considered to be the result of subtracting an optical spot 503, which is formed by a beam blocked by the blocking mask 403, from an optical spot 502, which is formed by the entire effective beam 402. The optical spot 501 at point P, the optical spot 502 formed by the entire effective beam 402, and the optical spot 503 formed by the beam blocked by the blocking mask 403 are shown in FIG. 5.
As can be seen from FIG. 5, the optical spot 501 at point P in FIG. 4 is narrower than the optical spot 502 formed by the entire effective beam 402 in the center region (hereinafter called the main lobe 504), but has troughs (hereinafter called the side lobes 505) on both sides. In FIG. 5, the side lobes 505 are negative in value, and optically this means that the optical phase is shifted by 180 degrees compared with that of the positive portions, that is, the optical phase is reversed. However, these side lobes 505 also have light intensities. As a result, an optical spot appearing to have three peaks is formed at point P. In FIG. 5, complex amplitude is plotted along the vertical axis and the position along the horizontal axis.
An optical spot having such three peaks poses a problem when it is applied to an optical disk pickup. That is, the surface of an optical disk is also illuminated with the two peak portions appearing on both sides, and the reflected light is detected as noise. A method for overcoming this problem is described in xe2x80x9cOpticsxe2x80x9d (Vol. 18, No. 12, pp. 691-692, 1989). This method proposes to selectively eliminate side lobes by using very fine slits in the light path. However, it has been extremely difficult to align the fine slits, because if the slit position is displaced, the main lobe is also blocked. Furthermore, adherence of dust to slit gaps has also been a problem. A further problem has been that diffraction of light occurs due to the presence of the slits.
Accordingly, it is an object of the present invention to provide an optical device that solves the above problems and that can easily eliminate only side lobes or side lobe components from a super-resolution optical spot.
It is another object of the invention to provide an optical device that can switch between super resolution and normal resolution by using a simple method.
To attain the above objects, the present invention provides the following configuration.
The optical device of the invention comprises: blocking means for blocking a portion of a first linearly polarized light; a polarizing beam splitter for transmitting the first linearly polarized light. therethrough, and for reflecting the second linearly polarized light whose plane is oriented at right angles relative to the first linearly polarized light; a quarter-wave plate for converting the first linearly polarized light into first circularly polarized light and the first circularly polarized light into the first linearly polarized light, while converting the second circularly polarized light, whose plane is rotating in a direction opposite to the first circularly polarized light, into the second linearly polarized light; a first converging lens for forming an optical spot on the optical disk, and for allowing reflected light from the optical disk to pass therethrough in a backward direction; and a second converging lens for focusing a beam of light reflected by the polarizing beam splitter onto a light detector.
In another aspect of the invention, the optical device comprises: a half-wave phase shift mask for shifting the phase of a portion of a first linearly polarized light by a half wavelength; a polarizing beam splitter for transmitting the first linearly polarized light therethrough, and for reflecting the second linearly polarized light whose plane is oriented at right angles relative to the first linearly polarized light; a quarter-wave plate for converting the first linearly polarized light into a first circularly polarized light and the first circularly polarized light into a first linearly polarized light, while converting the second circularly polarized light, whose plane is rotating in a direction opposite to the first circularly polarized light, into the second linearly polarized light; a first converging lens for forming an optical spot on the optical disk, and for allowing reflected light from the optical disk to pass therethrough in a backward direction; and a second converging lens for focusing a beam of light reflected by the polarizing beam splitter onto a light detector.
In a further aspect of the invention, the optical device comprises: an optically active device for converting a portion of a first linearly polarized light into a second linearly polarized light whose plane is oriented at right angles relative to the first linearly polarized light; a polarizing beam splitter for transmitting the first linearly polarized light therethrough, and for reflecting the second linearly polarized light whose plane is oriented at right angles relative to the first linearly polarized light; a quarter-wave plate for converting the first linearly polarized light into a first circularly polarized light and the first circularly polarized light into the first linearly polarized light, while converting a second circularly polarized light, whose plane is rotating in a direction opposite to the first circularly polarized light, into the second linearly polarized light; a first converging lens for forming an optical spot on the optical disk, and for allowing reflected light from the optical disk to pass therethrough in a backward direction; and a second converging lens for focusing a beam of light reflected by the polarizing beam splitter onto a light detector.
Preferably, the optical spot comprises a main lobe of the first circularly polarized light and a side lobe of the second circularly polarized light, and the reflected light comprises reflected light having the second circular polarization caused by the main lobe and reflected light having the first circular polarization caused by the side lobe.
Also preferably, the quarter-wave plate converts the reflected light of the main lobe into the second linearly polarized light and the reflected light of the side lobe into the first linearly polarized light.
Further preferably, the second converging lens focuses, on the light detector, the reflected light of the main lobe having the second linearly polarized light reflected by the polarizing beam splitter.
Preferably, the blocking means, the half-wave shift mask, or the optically active device is disposed between the optical spot forming means and the polarizing beam splitter.
Using simple configuration, the side lobe components peculiar to super resolution can be removed from the reflected light.
Since slits or the like are not used for removal of the side lobe components peculiar to super resolution, the need for alignment of slits, etc. is eliminated.
Furthermore, the use of the polarizing beam splitter ensures efficient utilization of light because incident light is not separated unnecessarily.